It is well known in the art that the essence of high fiber tensile strength and modulus is the capability of macromolecules, of adequately high molecular weight, to adopt a fully extended (rod-like) conformation and good parallelization (orientation) with respect to the fiber axis. With fairly stiff-chain para-aramids, exemplified by poly(p-phenyleneterephthalamide) (PPD-T), good orientation is accomplished by virtue of the ability of precursor liquid crystalline spinning solutions to become well oriented on extrusion through a spinneret and prior to coagulation. Fibers thus formed are not capable of significant draw-orientation, although modulus, and sometimes, to a lesser extent, tenacity may be improved by heat-treatment under tension, which is in effect a crystallization process. A second route to oriented aramid fibers of high strength and modulus is by choice of a semi-flexible composition such that, while it cannot form ordered, liquid crystalline solutions, an initially amorphous as-spun fiber may be very highly stretched at elevated temperature to bring the chains into good alignment. These are typically random copolymers which are non-crystalline in the drawn state. Random copolymers based on the corresponding diacid, 3,4'-oxydibenzoic acid have been disclosed in a general way, e.g., as part of a large number of possibilities for combining certain cited monomers (Japanese Patent Application 78-143726), or as within the description of a generalized formula (H. Sasaki et al., U.S. Pat. No. 4,507,467). Such drawable copolymers generally do not have the capability for substantial crystallization, which is known, as cited earlier, to enhance orientation which is a major prerequisite for maximization of fiber strength.
Herein are disclosed random copolymers based on terephthalic acid, mono- or dichloroterephthalic acid, 4,4'-bibenzoic acid or 2,6-naphthalene dicarboxylic acid, and p-phenylenediamine (PPD), which upon drawing as spun fibers at high temperatures, or just heating as spun fibers under tension yields fibers with a combination of high tensile strength, high modulus, and relatively high elongation to break.
U.S. Pat. No. 4,698,414 discloses copolymers derived from terephthalic acid, 2,6-naphthalene dicarboxylic acid, and PPD. U.S. Pat. No. 5,003,036 discloses polymers containing units derived from chloroterephthalic acid, a second dicarboxylic acid such as terephthalic acid, and PPD. Neither of these patents mentions a polymer containing both chloroterephthalic acid and 2,6-naphthalene dicarboxylic acid. Fibers made in both patents cannot be drawn significantly in the dry state.
N. V. Luikasheva, et al., Vysokomol. Soedin., Ser. B, Vol. 20, p. 151-154 (1978) reports the synthesis of a polymer containing units derived from terephthalic acid, 4,4'-bibenzoic acid and PPD. It is believed that chloroterephthalic acid is not mentioned in this paper.